A plain guide to antifreeze, from 1920s alcohol mixes and the DEX-COOL dispute to today's glycol coolants and how to test them.
Antifreeze lowers the freezing point of water, and common types raise its boiling point, so engines can work in cold and hot conditions. Ethylene glycol and propylene glycol are the main bases, and most formulas include corrosion inhibitors and a dye.
Antifreeze is the chemistry that keeps engines and heating systems working in the cold. Mixed with water it forms the coolant that carries heat away, and the same principles now serve HVAC chillers, solar water heaters and electronics.
What is antifreeze and how does it work?
Antifreeze is an additive that lowers the freezing point of a water-based liquid, a process known as freezing-point depression. Common antifreezes also raise the boiling point, which allows a higher coolant temperature. Their mixture with water keeps the liquid in its working phase across a wide temperature range.
Water has useful properties as a coolant. It is cheap, nontoxic and has a high heat capacity, but it has only a 100 Kelvin liquid range and it expands when it freezes. Antifreeze additives have lower heat capacities than water, so adding them reduces water's cooling ability, which is the trade-off engineers accept.
Commercially, the concentrate and the diluted mixture are both called antifreeze. Most formulations for heat transfer also include anti-corrosion and anti-cavitation agents, which protect the hydraulic circuit from progressive wear.
Antifreeze lowers the freezing point of water and raises its boiling point, but its inhibitors need regular replacement.
Why do engines need antifreeze?
Water makes a good coolant, but on its own it causes problems. If engine coolant boils inside the engine it forms steam pockets, which create hot spots and can lead to catastrophic failure. If plain water freezes in a northern climate, it can cause significant internal engine damage.
Plain water also increases the prevalence of galvanic corrosion. A proper engine coolant and a pressurised cooling system overcome these shortcomings. With 50% propylene glycol diluted in distilled water and a 15 psi system, the coolant can tolerate temperatures from -34 F (-37 C) to +265 F (129 C).
A pressurised system is part of the fix, alongside the right antifreeze, because each addresses a different shortcoming.
Automotive coolant also contains corrosion inhibitors, because radiators often include a mix of electrochemically incompatible metals such as aluminium, cast iron, copper, brass and solder. The coolant protects them all at once, which is why the choice of formula matters.
How did antifreeze chemistry develop?
Alcohols such as methanol, ethanol and ethylene glycol have been the basis of all antifreezes since they were commercialised in the 1920s. Salts lower freezing points too, and they are used for de-icing, but they corrode metals and cannot be used in cooling systems.
Early engine coolant antifreeze was methanol. Ethylene glycol was developed because its higher boiling point suited heating systems. Solutions first became available in 1926 and were marketed as permanent antifreeze, since they helped in summer as well as in cold weather.
Glycerol was also used for automotive antifreeze before ethylene glycol replaced it. Volkswagen introduced G13 antifreezes containing glycerol in 2008, marketed as better for the environment, but since 2018 it has moved on to G12evo, which no longer contains glycerol.
What are the main antifreeze types?
Most products start from a base of ethylene glycol or propylene glycol, which is blended with water and additives. Propylene glycol is considerably less toxic and is used where ethylene glycol would be inappropriate. The table compares the main bases and their common uses.
| Type | Key property | Typical use |
|---|---|---|
| Ethylene glycol (MEG) | High boiling point, low freezing point, high specific heat and thermal conductivity; toxic | Automotive engines and HVAC systems |
| Propylene glycol (MPG) | Considerably less toxic; sometimes sold as non-toxic antifreeze | Food-processing systems, home water pipes and electronics cooling |
| Methanol | The earliest engine coolant antifreeze | Superseded by ethylene glycol |
| Glycerol | Non-toxic, withstands high temperatures and is non-corrosive | Once used in cars; still mandated in many sprinkler systems |
| Propylene glycol methyl ether | More volatile than glycol | Antifreeze in diesel engines |
Ethylene glycol also has a low viscosity, which reduces pumping requirements. Propylene glycol can be used in enclosed spaces and is generally recognised as safe for food applications, which is why it is common in places where incidental contact is possible.
What do corrosion inhibitors and dilution do?
Most commercial antifreeze includes corrosion inhibitors and a coloured dye to aid identification. Those inhibitors protect the cooling system over time, and they are the part that wears out first. A 1:1 dilution with water is usual, giving a freezing point of about -34 F (-37 C) depending on the formulation.
Weaker or stronger dilutions suit warmer or colder areas. A water-to-antifreeze ratio of 40/60 to 60/40 is often specified to ensure corrosion protection, while 70/30 gives maximum freeze prevention down to -84 F (-64 C). The mix must be balanced against the job it has to do.
Hybrid formulas mix an OAT base with a traditional inhibitor, usually silicates. Zerex G-05 is a low-silicate, phosphate-free formula that includes a benzoate inhibitor. Phosphate HOAT is typically used in Asian makes and is often dyed red or blue, while silicate HOAT is used in European makes and is often dyed pink.
Traditional inhibitors were silicates and phosphates. American makes used both, European makes used silicates but no phosphates, and Japanese makes used phosphates but no silicates. Organic acid technology, or OAT, later changed that picture.
Organic acid technology antifreeze, such as DEX-COOL, was designed for a longer service life than the older mixtures. and hybrid organic acid technology (HOAT), such as Zerex G-05, are claimed to last five years or 240,000 km (150,000 mi). HOAT life expectancy can be as high as 10 years or 180,000 miles.
What was the DEX-COOL controversy?
DEX-COOL became the subject of litigation over intake manifold gasket failures on General Motors' 3.1L and 3.4L engines, with further failures reported on 3.8L and 4.3L units. One of its anti-corrosion components, presented as a 2-ethylhexanoate, was incompatible with nylon 6,6 and silicone rubber, and acted as a plasticiser.
Class action lawsuits were registered in several US states and in Canada. The first to reach a decision was in Missouri, with a settlement announced in early December 2007. In late March 2008 GM agreed to compensate complainants in the remaining 49 states, and GM later filed for bankruptcy in 2009, which tied up the outstanding claims.
GM's internal documents point to long spells of driving with low coolant as the root problem. Pressure caps that failed in the open position let coolant escape, and iron oxide contamination then held the caps open permanently, creating a cycle that was hard to break.
The lesson for owners is practical. Check the coolant specification in the handbook before changing formulas, and do not mix types unless the maker says they are compatible.
Honda and Toyota's extended-life coolants use OAT with sebacate but without 2-EHA, and some added phosphates to protect the system while the OAT builds up. Honda specifically excludes 2-EHA from its formulas.
How is antifreeze tested and maintained?
Antifreeze needs periodic checking once it is in use, and the reading tells a technician whether the mixture will survive the coming weather. Three methods measure the concentration and so the freeze point: specific gravity with a hydrometer or floating indicator, a refractometer that reads the refractive index, and disposable test strips.
Propylene glycol cannot be tested by specific gravity, because 40% and 100% solutions give the same reading. Typical uses rarely exceed 60% concentration, so refractometers and test strips are the practical tools for that coolant.
Without leaks, the base antifreeze chemicals can retain their properties for a long time, but corrosion inhibitors are gradually used up and must be replenished. Most automotive makers therefore recommend a periodic full replacement of coolant to renew the inhibitors and remove contaminants.
Maintenance checks also cover inhibitor level, colour and biological contamination, because bacterial slime speeds up corrosion once it starts to grow. Propylene glycol should be replaced when it turns reddish. That colour indicates that iron in the system is corroding significantly, and without inhibitors the glycol can oxidise into organic acids that speed up metal corrosion.
Is antifreeze dangerous?
Ethylene glycol is poisonous to people and other mammals, and it can be fatal. It is metabolised in the liver into oxalic acid, which can cause kidney failure, so dialysis is a major treatment. Early symptoms include vomiting, confusion, abdominal pain, agitation and ataxia, and long-term damage can include kidney, brain and nervous system harm.
Antifreeze can attract children and pets because of its sweet taste, and it is sometimes drunk as a substitute for alcohol. Many formulations include bitter additives to discourage drinking, although some studies do not support embittering as a way to reduce ingestion, so spills should be cleaned up and containers kept well out of reach.
Propylene glycol has a very different profile. The US Food and Drug Administration allows it in many ultra-processed foods, including ice cream and baked goods, and it is the main ingredient in e-liquids used in electronic cigarettes. Its low toxicity is the reason it is chosen wherever incidental ingestion is possible.
Treatment within an hour of ingestion makes a far better outcome likely, so anyone who may have swallowed antifreeze needs urgent medical help straight away.
Where else is antifreeze used?
Water with ethylene or propylene glycol is the most common mixture in electronics cooling. Ethylene glycol solutions formulated for cars often contain silicate rust inhibitors that can coat or clog heat exchanger surfaces, so the choice depends on the application.
Similar mixtures are used in HVAC and industrial heating and cooling systems as high-capacity heat transfer media. Many of those formulations include corrosion inhibitors that are replenished to keep expensive piping and equipment from corroding.
Some organisms produce antifreeze proteins that bind small ice crystals and stop them growing, which lets those organisms operate well below the freezing point of water. In cryobiology, cryoprotectants such as ethylene glycol, propylene glycol and glycerol help protect sperm, blood, stem cells and plant seeds from freezing damage.
